Asymmetric membranes for use in a variety of purification operations have long been known. Permeation selective or permselective membranes preferentially pass certain components of liquid mixtures while acting as a barrier for other components. In reverse osmosis membranes, a hydrostatic pressure in excess of the equilibrium osmotic pressure of the liquid mixture or solution is applied to the liquid to force the more permeable components, usually water, through the membrane in preference to the less permeable components, such as a salt, contrary to the normal osmotic flow.
In the preparation of membranes for these applications, a film or hollow fiber is first cast from a casting dope. After partial removal of the casting solvent, this film is then extracted by treating with a nonsolvent to replace the balance of the original casting solvent. A wide variety of additives has been suggested for the modification of the physical and performance characteristics of the resulting membrane. For example, various salts such as lithium chloride and lithium nitrate have been used to increase the permeability of the resulting membrane. However, while these have functioned satisfactorily in the modification of the final product, the salts typically have the undesirable characteristic of decreasing the salt rejection of the resulting membrane. In addition, some lithium salts have resulted in decreased thermal stability of casting solutions containing them.
Such asymmetric membranes of aromatic polyamides (polyaramides) are known, e.g., from U.S. Pat. No. 3,567,632.
It is an object of this invention to provide improved asymmetric membranes of aromatic polyamides, improved casting dopes of such polymer, and processes for making same.
It is a more specific object of this invention to provide a casting dope in N,N-dimethylacetamide which contains a substantial amount of magnesium chloride.
It is a further object of this invention to provide a casting solution from which membranes of consistently high salt rejection can be made.